I've been shooting AR-15s and M-16s for over 15 years and just had a problem I had never seen or heard of.

I have an M-4 Copy that I built from a DPMS Lower and a J&T Upper Mock M4 Parts Kit. The stock is a "pinned open" carbine stock. Friday I was shooting and noticed the bolt wouldn't hold open and I couldn't pull it back. After troubleshooting, I decided to pull out the Recoil Buffer and spring which turned out to be the problem. The buffer (standard shorty buffer) had come apart in the stock. This is not a new rifle (probably has 1500 rounds through it) and so I thought it might have just broken. I put it back together and tried again. This time it came apart within 3 rounds. So I thought this thing is just broken. Well I went and bought another buffer and replaced it. And within 10 rounds the same thing happened. I asked a few guys at the range and no one had ever heard of this before.

The only difference between this trip and the last was I was shooting some new ammo I reloaded. So I thought maybe I loaded it "hot". I put the buffer back together and filled up the mag with some Spanish surplus and after about 20 rounds the same thing happened. BTW, based on recoil my reloads are definitely not hot, but probably light. Could this cause the problem.

The buffer is an outer casing/housing, with tungsten weights in the housing (casing) to act like a dead blow hammer and the end urethane bumper held in with a roll pin.
At what location did the buffer break apart?
Is the end bumper being chewed up by something in the end of the receiver extension, and what type of buffer are you using. Is it the BB type or tungsten weight type, the standard car buffer, or the plastic ended 9 SMG type.
Bottom line is in order for the rifle to recoil the buffer hard enough against the back of the receiver extension would require the ammo to be loaded hot as hell, But the rub to this that when the ammo is too hot, the case sticks in the chamber due to the higher pressure. This leave three areas to check on the rifle. The first would be the buffer itself, the two may be cheap parts that are defective. The second would be the recoil spring, which may be weak or clipped. The last would be the bolt lock up, or lack of that is causing the problem.
Also, are you sure that your stock uses the shorter car buffer, and not the full-length size? Using the wrong buffer would cause free run for the buffer, and the carrier key to slam into the back of the charging handle, which leads to a cracked receiver.
If possible, post a photo of the broken buffer so we can get an idea of the breakage, and what may be causing the problem.

I once thought that I had all the answers, but after consorting with the "Magic 8 Ball", I have found it to be "Doubtable Not So".

They are the type with the tungsten weight inside. Also neither has a roll pin like a full size buffer. (I was actually thinking about drilling them and adding a pin).
I should also clarify that they aren't really breaking, but coming apart. The rubber cap comes off and the pieces fall out into the stock. All I have to do is put them back together, of course doing that after every 3-6 rounds is "not expected behavior" :)
As for the spring its the spring the that came with the stock. What's really weird is I've probably fired 1500 rounds of various surplus ammo through the rifle and had no issues.
As for my reloads. They were a light load and I had no stuck cases or other indicators of the rounds being loaded to hot.
I'll try to take a picture of the buffer tomorrow.
Thanks for the reply.

They came out of a couple of generic CAR stocks. I think you're thinking what I was thinking...cheap parts. Its just weird the first one lasted through so many rounds and the second one only lasted through about 6. Peculiar??